Support sqlite3 on Linux

[BCSharp]

Part of #1483.

[BCSharp]

I tried a little to get it work on macOS but it was too much effort. If I had the time to work more on sqlite3, I'd rather replace the backend than spend time hacking the current code that is already heavily hacked.

[slozier]

Little bit odd to have isNT returning true on Unix, but the code is already a mess, so looks good to me.

[BCSharp]

Little bit odd to have isNT returning true on Unix, but the code is already a mess, so looks good to me.

I agree and actually had a commit changing it to isNTorPosix but it did not look cleaner: a lot of code calling isNT is commented out so either the rename would leave it using a function that is no longer there or if comments were renamed too then the code in those block would really look weird as it is doing sometimes some pretty non-Posix stuff. So in the end, I opted for a minimal change hence no rename.